Using the Camera Data Node
Shading Point

is the nearest point of intersection of the camera ray with a visible object, which the render engine is currently calculating the color for (these calculations are referred to as shading)



The real View Vector

it is a vector (or a ray) in the camera space, starting from the center of the camera (or the view) and reaching the shading point 





View Vector

it is a normalized view vector (magnitude = 1 or unit vector) in the camera space, starting from the center of the camera (or the view) and pointing at the shading point (it doesn't necessarily reach it )





View Z Depth


it is the Z component of the view vector (the non normalized)
it is the shortest distance from the shading point to the camera's XY plane (the is not normalized Z-depth, it is not from 0 to 1)






View Distance

it is the magnitude of the non normalized view vector, or simply the distance from the shading point to the camera's center 





where to use each one: well you know enough now to decide where to use them!